(a) Field of the Invention
The present invention relates to a semiconductor wafer and a method for fabricating a semiconductor wafer and, more particularly, to an improvement of the semiconductor wafer for preventing ingress of particles generated by grinding the bottom surface of the wafer.
(b) Description of the Related Art
Some semiconductor wafers are formed by a process including the steps of forming polyimide overcoat film patterns for protecting the main surfaces of semiconductor chips and grinding the bottom surface of the wafer after the polyimide film patterns are formed on the main surfaces.
A conventional method of fabricating such a semiconductor wafer will be described with reference to FIGS. 1 and 2. FIG. 1 is a top plan view showing a part of the main surface of the conventional semiconductor wafer having thereon a polyimide overcoat film pattern on each of the semiconductor chips. FIG. 2 is a sectional view taken along line IIxe2x80x94II in FIG. 1, showing the wafer onto which a surface protective tape 19 is stuck after formation of the polyimide film patterns 18.
The semiconductor wafer has thereon a plurality of semiconductor chips 11 arranged in a matrix. The fabrication process of the wafer shown includes the step of forming insulating films 14 on the substrate 13, followed by forming thereon an interconnect pattern 16 including electrode pads 15. Scribe lines 17 are then formed between each adjacent two of the semiconductor chips 11. The scribe lines 17 is used for cutting the wafer in a later dicing step in which the semiconductor wafer is scribed and separated into separate semiconductor chips 11.
Polyimide film patterns 18 are formed as overcoat layers for protecting the interconnect patterns 16 against damages or contamination. The polyimide film forming step typically includes such procedures as polyimide coating, polyimide hardening, resist coating, exposure to light, selective removal of the resist layer for patterning, selective etching of the polyimide film using the resist layer as a mask, and removal of the resist layer.
For patterning the polyimide film, the polyimide film is removed from most of the areas for all the electrode pads 15, the areas of the scribe lines 17, and, if any electrode pads 15 and scribe lines 17 are disposed in close proximity to one another, the gaps therebetween. The purpose of the removal of the polyimide film from the areas of the scribe lines 17 is to prevent the dicing blade used in the dicing step from a premature deterioration in sharpness. The areas of the electrode pads 15 should be exposed for the sake of bonding in a bonding step, and thus are subjected to the removal of the polyimide film.
The polyimide film patterns 18 have a certain thickness so as to protect the interconnect patterns 16 from damages and contamination. The polyimide film patterns 18 are formed in a thickness greater than or equal to 6 xcexcm, which is significantly greater as compared with those of the insulating films 14 and interconnect patterns 16 which are not greater than 2 xcexcm. Thus, the surface of the wafer having thereon the polyimide film patterns 18 is projected where the polyimide film patterns 18 exist, and depressed where no polyimide film 18 exist. Parallel to the scribe lines 17, these depressions extend vertically and horizontally to form a lattice. Further, if any electrode pads 15 and scribe lines 17 are disposed in close proximity to one another and thus no polyimide film 18 is formed therebetween, the depressions over those electrode pads 15 and the depressions over those scribe lines 17 are coupled together. These depressions create gaps 20, for example, between the wafer and the surface protective tape 19 which is stuck onto the wafer during grinding the bottom surface of the wafer.
FIG. 3 is a side view showing the grinding position in a bottom surface grinder 21 for use in the step of bottom surface grinding in the fabrication process of the semiconductor wafer. The bottom surface grinder 21 has a suction table 22 and a high-speed rotation wheel 23 in its grinding position. The wafer. 12 having thereon the polyimide film patterns onto which the surface protective tape 19 are stuck is attached to the suction table 22, with the main surface (top surface) of the wafer 12 being directed downward. The high-speed rotation wheel 23 rotates at a high speed to grind the bottom surface of the wafer 12. The high-speed rotation wheel 23 has a plurality of grindstones 25 protruding therefrom.
Nozzles 26 for spraying water are arranged right inside the grindstones 25. Aside from the grinding position, the bottom surface grinder 21 has a cleaning position (not shown) for cleaning particles 24 produced by the grinding. The bottom surface grinder 21 is used to grind the bottom surface of the semiconductor wafer 12 to a desired thickness in the following manner.
That is, the wafer 12 having the surface protective tape 19 stuck thereon is attached to the suction table 22 with the top surface of the wafer 12 being directed downward. The high-speed rotation wheel 23 having the grindstones 25 is lowered to the height of the bottom surface of the wafer 12 and rotated at a high speed, so as to grind the bottom surface of the wafer 12. Formed inside the grindstones 15 are the nozzles 26 for ejecting water to wash away the ground particles 24 produced by the grinding and to cool down the frictional heat produced by the grinding. The nozzles 26 eject water to wash away the ground particles 24. There are some other methods which includes a grinding step without the supply of water. After the bottom surface grinding, the wafers are cleaned in the cleaning position (not shown), drained, and stored in a storage box in the order of the appearance.
In the conventional method in which all the steps up to the patterning of the polyimide film are completed before the bottom surface grinding, since the polyimide film is already removed from the areas for the electrode pads 15 and the scribe lines 17 to make depressions, the gaps 20 are generated between the surface of the wafer 12 having thereon the polyimide film patterns 18 and the surface protective tape 19.
Due to the presence of the gaps 20, the particles 24 produced by the bottom surface grinding, mixed with the water or air and washed around from the bottom surface to the side surface of the wafer 12, may intrude into the interior of the wafer through gaps 20 from the periphery of the wafer. Moreover, by the capillary function, the water/air containing the ground particles 24 flows onto the main surface of the wafer along the scribe lines 17 which extend vertically and horizontally to form a lattice and are in connection with one another.
On the way of flow, the ground particles 24 adhere to the electrode pads 15 having depressions which lead to the scribe lines 17. Of the ground particles 24, hard and pointed ones stick into the electrode pads 15, which are formed of relatively soft material. This wafer is cleaned in the cleaning position (not shown) after the bottom surface grinding, whereas the ground particles 24 intruding into the gaps 20 and adhered thereto are eliminated insufficiently. Although there is another cleaning step (not shown) after the removal of the surface protective tape 19, the ground particles 24, sticking into the electrode pads 15, are hard to remove completely.
Patent Publication JP-A-61-232625 describes another method including the steps of preparing a wafer having thereon a polyimide film covering the entire surface of the wafer including the scribe lines and electrode pads, grinding the wafer in the bottom surface grinding step, and entirely removing the polyimide film by etching. This technique eliminates the problem that the pasting wax for pasting the wafer to the table of the bottom surface grinder reacts with the electrode pads to discolor the electrode pads. In addition, a problem of damage to the electrode pads can also be avoided. Moreover, even in the cases where the wafer on which the interconnect patterns are formed has silver bump plating layers having projections and depressions of 20-50 xcexcm on its surface, the surface of the wafer is flattened by coating the polyimide film having a greater thickness.
This prevents generation of cracks in wafer due to the projections and depressions on the surface of the wafer when the wafer is pasted during the bottom surface grinding.
In the conventional fabrication method described in the patent publication mentioned above, the entire surface of the wiring-patterned wafer is covered with the polyimide during the bottom surface grinding, and thus produces no projections/depressions due to the presence/absence of the polyimide film patterns. This prevents ground particles from adhering to the electrode pads, and the polyimide film is removed after the bottom surface grinding step.
However, the described technique is not directed to a wafer having thereon polyimide film patterns for protecting the product semiconductor device, because the polyimide film is used as a temporary protective film and entirely removed from the wafer after the bottom surface grinding. The present invention is directed to solving the problem involved in the first conventional method wherein the bottom surface grinding is conducted after patterning of the polyimide overcoat film.
An object of the present invention is to provide a process for fabricating a semiconductor wafer including a bottom surface grinding step, which is capable of preventing the ground particles produced in the bottom surface grinding step from contaminating the electrode pads formed on the semiconductor wafer.
The present invention provides a method for fabricating a semiconductor wafer, including the consecutive steps of forming a wafer having thereon a plurality of semiconductor chips arranged in a matrix and a plurality of scribe lines separating the semiconductor chips from one another, each of the semiconductor chips including an interconnect pattern having electrode pads, forming an overcoat film pattern on each of the chips except for at least effective areas for the electrode pads, the overcoat film pattern having a bank portion bridging the overcoat film pattern and an adjacent overcoat film pattern while crossing one of the scribe lines, and grinding a bottom surface of the wafer.
The present invention also provides a semiconductor wafer comprising: a plurality of semiconductor chips arranged in a matrix and each having an interconnect pattern including electrode pads and an overcoat film for covering the semiconductor chip except for at least parts of the electrode pads; and a plurality of scribe lines for separating the semiconductor chips from one another, at least one of the overcoat films having a bank portion bridging the at least one of the overcoat films and an adjacent one of the overcoat films while crossing one of the scribe lines.
The semiconductor wafer according to the present invention and the semiconductor wafer fabricated by the fabrication method according to the present invention can suppress the ingress of ground particles by the function of the bank portions through the scribe lines during the bottom surface grinding step, and prevent the electrode pads from being contaminated or damaged by the ground particles. This reduces the failures of the wafer to be found in the following inspection step, and allows a safe bonding process for the electrode pads.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.